The present invention relates to a shock-absorbing material suitable for use in a core material for automotive bumpers, or the like.
At present, shock-absorbing materials used as core materials for automotive bumpers, and the like are made mainly of a synthetic resin foam. An automotive bumper making use of a synthetic resin foam is generally composed of a core formed of the synthetic resin foam, and a skin material made of a synthetic resin, with which the core is covered.
Shock-absorbing materials used as core materials for the automotive bumpers, and the like are generally required to satisfy 1) to have excellent energy absorption performance, 2) to have an excellent dimensional recovery factor and 3) to provide a low-density and light-weight material at the same time.
Japanese Patent Application Laid-Open Nos. 221745/1983 and 189660/1985 each disclose core materials for automotive bumpers, which satisfies the above three conditions.
These publications disclose polypropylene and ethylene-propylene copolymers as base resins for bumper cores.
Further, Japanese Patent Application Laid-Open No. 158441/1990 and Japanese Patent Application Laid-Open No. 258455/1995 disclose 1-butene-propylene random copolymers, and 1-butene-propylene random copolymers and ethylene-1-butene-propylene random terpolymers, respectively, as materials used for bumper cores.
When a shock-absorbing material used as a bumper core or the like is produced by using a polypropylene resin as a material, there is generally used the so-called bead molding process in which foamed particles are filled into a mold and heated to expand the foamed particles, thereby mutually fusion-bonding them to obtain an expansion-molded article conforming to the mold. The expansion-molded article of the polypropylene resin particles obtained by this process is excellent in shock-absorbing property and impact resilience and has excellent physical properties such as light weight and small residual strain.
Accordingly, the shock-absorbing material composed of the expansion-molded article of the polypropylene resin particles has excellent properties compared with shock-absorbing materials composed of other resin materials. However, its stiffness and energy absorption efficiency are not necessarily satisfactory, and so it yet leaves room to improve. Raw resins actually used for producing shock-absorbing materials at present are propylene copolymers. A polypropylene homopolymer itself is a high-stiffness polymer and most suitable for use as a raw resin for producing a shock-absorbing material. On the other hand, such a resin has involved problems that its molding temperature becomes high due to its high melting point, and the molding temperature range for successfully expanding it is limited due to its viscoelastic property. As described above, the use of the polypropylene homopolymer as the molding resin has involved the difficulty of presetting the optimum conditions upon molding and hence a problem that defective fusion bonding among resin particles is caused by, for example, a slight error in temperature setting. Therefore, this resin has been poor in moldability.
The reason why the propylene copolymers are actually used as molding resins is that their moldability is better compared with the polypropylene homopolymer.
However, the propylene copolymers naturally has the demerit that their stiffness is low, and so some attempts have been made to improve the stiffness by lessening the content of other component(s) than propylene in the copolymers. However, only unsatisfactory results have been obtained. In addition, any shock-absorbing material comprising a propylene copolymer as a base resin is unsatisfactory even from the viewpoint of energy absorption efficiency.
The investigation by the present inventors revealed that when a polypropylene homopolymer (hereinafter referred to as xe2x80x9cthe metallocene PPxe2x80x9d) obtained by using a metallocene polymerization catalyst is used as a molding resin, a shock-absorbing material having good physical properties is obtained.
However, it was also confirmed that in order for a shock-absorbing material to achieve high stiffness and energy absorption efficiency, the mere use of such a polypropylene homopolymer as a base resin is insufficient, and other factors than this must be added. Thus the present inventors have carried out a further investigation. As a result, it has been found that when the tensile modulus of the metallocene PP and the quantity of heat at a high-temperature peak appeared on a DSC curve obtained by the differential scanning calorimetry of the resulting molded article are defined within specific numerical ranges, a shock-absorbing material having high stiffness and energy absorption efficiency can be obtained. The present invention has been led to completion on the basis of this finding.
It is an object of the present invention to provide a shock-absorbing material used as a bumper core or the like, which is produced by using a polypropylene homopolymer as a molding raw resin.
Another object of the present invention is to provide a shock-absorbing material excellent in stiffness and energy absorption efficiency compared with the conventional materials.
A further object of the present invention is to provide a shock-absorbing material which imparts the advantage in production conditions that the pressure of steam fed into a mold upon molding can be controlled low.
The present invention relates to a shock-absorbing material composed of an expansion-molded article produced by using foamed particles comprising a metallocene PP as a base resin, wherein the base resin has a tensile modulus of at least 15,000 kgf/cm2, and the expansion-molded article has a crystal structure that an inherent peak and a high-temperature peak appear as endothermic peaks on a DSC curve obtained by the differential scanning calorimetry of the molded article. The term xe2x80x9chigh-temperature peakxe2x80x9d as used herein means a peak appeared on the temperature side higher than a temperature corresponding to the inherent peak of endothermic peaks appeared on a DSC curve obtained by heating 2 to 4 mg of a specimen cut out of the molded article to 220xc2x0 C. at a heating rate of 10xc2x0 C./min by means of a differential scanning calorimeter.
The foamed particles of the metallocene PP used in the production of the shock-absorbing material according to the present invention are those having a crystal structure that an inherent peak and a high-temperature peak appear as endothermic peaks on a DSC curve obtained by the differential scanning calorimetry of the foamed particles, and a quantity of heat of at least 25 J/g at the high-temperature peak.
When molding is conducted by using such foamed particles, the above-described crystal structure does not disappear, and so a molded article produced also has a similar crystal structure that an inherent peak and a high-temperature peak appear as endothermic peaks on its DSC curve. Further, the quantity of heat at the high-temperature peak in the molded article also indicates almost the same value as the quantity of heat at the high-temperature peak in the foamed particles, and its numerical value is at least 25 J/g.
The shock-absorbing material according to the present invention features that it is composed of an expansion-molded article of foamed particles comprising, as a base resin, a polypropylene homopolymer obtained by using a metallocene polymerization catalyst, the base resin has a tensile modulus of at least 15,000 kgf/cm2, the expansion-molded article has a crystal structure that an inherent peak and a high-temperature peak appear as endothermic peaks on a DSC curve obtained by the differential scanning calorimetry of the molded article, and the quantity of heat at the high-temperature peak in the molded article is at least 25 J/g. The fact that the shock-absorbing material has these three factors brings about an effect of markedly improving stiffness and energy absorption efficiency compared with the shock-absorbing materials composed of an expansion-molded article comprising the conventional polypropylene copolymer as a base resin.
The expansion molded article has compression stress of at least 7.28 kgf/cm2 and an energy absorption efficiency of at least 72.4 as reported in the below Table 2, and as illustrated in FIG. 3A and in FIG. 3B, respectively.
When the polypropylene homopolymer (metallocene PP) obtained by using the metallocene polymerization catalyst and used as a raw material for producing the shock-absorbing material according to the present invention is compared with a polypropylene homopolymer obtained by using a Ziegler-Natta catalyst, the metallocene PP characteristically has a lower melting point when both polymers have the same tensile modulus. As a result, when molding is conducted by using foamed particles comprising the metallocene PP as a base resin, the pressure of steam fed into a mold can be controlled lower, so that the following effects can be brought about. Namely, the consumption of heat can be decreased to reduce production cost, and the durability of the mold can be enhanced.